Method of heat-treating electrical resistance alloy



trical resistance.

Patented Feb. 12, 1952 METHOD OF HEAT-TREATING ELECTRICAL RESISTANCE ALLOY Victor 0. Allen'Madison, N. Jgassignorto Wilbur B. Driver Company, Newark; N; J., a.cor-.. poratiou of New Jersey No Drawing. Application- August 16; 1949, Serial'No. 110,671

9 Claims (01. 148-13) This invention relates to nickel chromium alloys o-f the type employed as electrical resistance elements and more particularly to a method of processing such alloys and electrical resistance wire comprised thereof to materially improve the electrical resistance and temperature coefiicient of resistance properties of such alloys, and this application isacontinuation-impart of application-serial No. 562,901, filed November 10., 19.44, now abandoned, which is assigned to the same assignee as the present application.

The object of the invention is to provide an electrical resistance element having a low temperature coefficient within the range 20 to 30.0 C'. and high electrical resistance which is suitablefor use in electrical devices requiring resistor'elements of high accuracy, such as radio; radar, television and the like devices.

Another object is to provide a methodof treatin' electrical resistance elements comprised ofia nickel chrom'ium electrical resistance alloy consisting of -30% Cr, balance mainly Ni, to imiprove the'normal temperature coefficient and electrical resistance properties of such alloys.

Still another object is to provide a method of treating electrical resistance wire comprised of an alloy consisting of 10% to 30% chromium, b'al-' ance mainly nickel, to condition the wire for use as an electrical resistor element in high accuracy electrical devices.

A further object is to provide a method of processing wire comprised of a nickel-chromium alloy of the 80/20 type; containing approximately chromium, balance mainly nickel, except for associated constituents not detrimental to the electrical resistance and temperature coefiicient properties of the alloy, to impart thereto a relatively low temperature coefiicient within the range 0 to 300 C. and a relatively high elec- Other objects will be apparent as the invention is more fully hereinafter disclosed.

In accordance with these objects I have discovered that when an electrical resistance alloy consisting of 10% to chromium, balance mainly nickel, is heat-treated for an extended time ranging from 1 to 5 hours at a temperature within the range 780 to 1000 F. followed by slow cooling to atmospheric temperatures,- the temperature coefiicient and electrical resistance 2 properties of the alloy are stabilized at values which are markedly lower and higher, respectively, than the heretofore accepted rated values normally present therein.

The reason for obtainin such an improvement in these properties in such alloys by such a heattreatment operation is not clearly apparent but it is believed that some internal rearrangement of atoms or grains resulting in-a stabilized crystal structure is primarily responsible for this im-e provement.

In the adaptation of this? discovery in the art of forming or producing. electrical resistor elements the nickel-chromium alloy is first reduced by hot and cold working methods common and well known in the art to a desired final size in accordance with prior art practice and is preferably; subjected-to the usual final annealing operation at a temperature within the range 1700-1950 F. for the required time interval effective to soften the wire structure by'recrystallization to the desired extent best adapting the wire product for forming into the desired type of electrical resistor element, such as a coil. In

most instances, the time and temperature employed in this final anneal operation is selected to yieldwhat is commonly known as a fully annealed' product for forming into an electrical resistance element, such as a.coil.c

In accordance with the present invention the mechanically deformed product, such as a wire, preferably-after such final annealing operation and prior to forming into the desired typeof electrical resistor element, such as a coil, is heattreated in an inert or reducing atmosphere, for from 1 to 5' hours at a temperature within the range 780 -1000 F., preferably at 850 F., and is permitted to cool slowly in the same inert atmosphere down to normal (room) temperatures from the heat-treating temperature. This heat-treatment operationmay be practiced under boxanneal conditions, common in the art,'or on succ'essive spools in a continuous feed and discharge furnace provided with heated and cooled sections.

As one specific example of the practice of the present invention, but not as a limitation of the same, the procedure't'o be followed in the heattreatment of a nickel-chromium alloy of theessentially binary /2O'type (20% Cr and '80 Ni) will be described.

Nickel-chromium alloys of the 80/20 binary type are, per se, old and well known in the art and are recognized generally as being the best of the many known types of nickel-chromium electrical resistance alloys. This type of alloy consists essentially of 20% chromium and 80% nickel but in the commercial production of this alloy it is not unusual to find fractional percentages of iron, manganese, silicon and carbon in the alloy displacing part of the nickel, the total amount of these constituents usually being held to under 2.% with Si approximating 1.00%, Fe .50% max, Mn .10% max, and C 05%. Occasionally iron as high as 1% will be present in such alloys. In some lower grades of this type of alloy considerably higher iron percentages are present.

In this standard type of high quality binary type of 80/20 nickel-chromium electrical resistance alloy, wherein the associated constituents are g held under about 2.0% the accepted and rated values for the temperature coeiiicient- (T. C.) and electrical resistance (E. R.) properties of drawn wire at final size usually approximates the following:

Annealed (at Hard Drawn R) T. C. (100 C.) -i 00018 00013 E. R .i 600 650 On heat-treatment of such wire products, in accordance with the present invention, at 850 F. in a hydrogenous reducing atmosphere for varying time intervals of from 1 to 3 hours the following T. C. and E. R. values were obtained:

HARD DRAWN In addition to the improvement in the T. C. and E. R. values an increase in tensile strength of about 10% was obtained in each instance.

A plurality of tests on other melts of similar composition 80/20 Ni-Cr alloys have indicated that as a result of such heat-treatment a T. C. value of approximately .0000? ohm and an electrical resistance of approximately 6'75 ohms (C. M. F.) can be consistently obtained. On longer heating at- 850 F. higher electrical resistance values are obtainable Without substantial change in the T. C. value, such that with 5 hours heating E. R. values approximating 700 ohms -(C. M. F.) may be consistently obtained.

Electrical resistors having a T. C. value under .0001 and an E. R. value above 650 are well suited for use in electrical devices and circuits of the high accuracy type, such as in radar, radio and television devices.

In Ni-Cr alloys of the 80/20 type it has heretofore been proposed to add to the alloy small percentages of other alloy constitutents such as Al, Al'Cu, Al-Ag, Al-Fe or Mn, to lower the T. C. value of the alloy in its annealed condition. The

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present heat-treatment operation on the annealed wire comprised of such low T. C. improvement alloys is effective to still further lower the T. C. value of these alloys closer to zero and by the term /20 nickel-chromium electrical resistance alloys as it hereinafter appears in the claims I mean to include such improvement alloys as being within the scope of the present invention.

In general I have found that as the temperature of heat-treatment is increased within the range 780-1000 F. the time at temperature may be decreased and vice versa to obtain equivalent results.

Having hereinabcve disclosed the invention generically and specifically and given one specific embodiment of the practice or" the same it is believed apparent that the invention may be widely varied and adapted for use on a plurality of nickel-chromium electrical resistance alloys without essential departure therefrom and all such modifications and adaptations of the same are contemplated as may fall Within the scope of the following claims.

What I claim is:

1. The method of treating a wire comprised of an alloy consisting of 10 to 30% Cr, balance Ni, to impart thereto a temperature coefiicient within the range 20 to 300 C. or" less than .0001 ohm and an electrical resistance above 650 ohms which comprises heat treating the wire in a reducing atmosphere for from 1 to 5 hours at a temperature within the range '780-1000 F. and cooling the heat-treated product slowly down to atmospheric temperatures.

2. The method of claim 1, wherein the said alloy contains iron, manganese, silicon and carbon in small fractional percentages and in total amount not exceeding 2 3. The method of claim 1, wherein the alloy consists of 20% Cr, balance Ni, the temperature of heating is 850 F., and the time interval of heating approximates one hour.

4. The method of claim 1, wherein'the heattreating is conducted in a hydrogenous reducing atmosphere at a temperature approximating 850 F.

5. The method of claim 1, wherein prior to said heat-treatment the wire is annealed at a temperature within the range 1700-1950 F.

6. The method of treating a wire comprised of an alloy consisting of 20% Cr, not over 2% of associated alloy constituents consisting of fractional percentages of at least one of the elements of the group consisting of Fe, Mn, Si and C, balance nickel, to impart thereto a temperature coeiTicient less than .0001 and an electrical resistance above 650 ohms and physical properties adapting the same to be formed into resistor elements for electronic electrical devices, which comprises annealing the wire at a temperature within the range 1700 to 1950 F. to condition the wire for forming, heat-treating the annealed wire in a hydrogenous reducing atmosphere for from 1 to 5 hours at a temperature within the range I80 to 1000 F., and slowly cooling the'annealed and heat-treated wire product to atmospheric temperatures in the same atmosphere.

7. The method of claim 6, wherein the said heat-treating temperature approximates 850 F. and the time interval of heating is 1 to 3 hours.

8. An electrical resistor element having a temperature coefiicient within the range 20 to 300 C. of less than .0001 and an electrical resistance above 650 ohms, said element being comprised of assume the annealed and heat-treated wire product of the method of claim 6.

9. An electrical resistor element comprised of the heat-treated wire product of the method of claim 1.

VICTOR 0. ALLEN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Bulletin T-7, published by International Nickel 00., New York, 1939, pages 5 and 8. 

1. THE METHOD OF TREATING A WIRE COMPRISED OF AN ALLOY CONSISTING OF 10 TO 30% CR, BALANCE NI, TO IMPART THERETO A TEMPERATURE COEFFICIENT WITHIN THE RANGE 20 TO 300* C. OF LESS THAN .0001 OHM AND AN ELECTRICAL RESISTANCE ABOVE 650 OHMS WHICH COMPRISES HEAT TREATING THE WIRE IN A REDUCING ATMOSPHERE FOR FROM 1 TO 5 HOURS AT A TEMPERATURE WITHIN THE RANGE 780-1000* F. AND COOLING THE HEAT-TREATED PRODUCT SLOWLY DOWN TO ATMOSPHERIC TEMPERATURES. 